Atoms mainly consist of what

3 Particles

Inside the Nucleus:

Protons = + 1 relative charge, relative mass of 1.

Neutrons = 0 relative charge, relative mass of 1.

Outside of Nucleus (On on shells):

Electrons = - 1 relative charge, relative mass of 0.0005.

Atoms are neutral since there is always the same number of protons as electrons, so the charges cancel each other out.